1. Field of the Invention
The present invention relates to a production method of a film-shaped optical waveguide for a connector which connects components (e.g., information processing components, information transmitting components and the like) to each other for light transmission between these components.
2. Description of the Related Art
Optical waveguides for connectors typically include an under-cladding layer, cores provided on a surface of the under-cladding layer and serving as optical paths, and an over-cladding layer provided over the cores.
On the other hand, where an optical waveguide having optical interconnections (cores) of a predetermined linear pattern is provided on a surface of an electric circuit board having electric wirings in an opto-electric hybrid board, the optical waveguide is produced in the following manner. As shown in FIG. 10A, an under-cladding layer 31 is formed on a surface of an electric circuit board 20 opposite from an electric circuit formation surface on which an electric circuit 21 is provided. In turn, as shown in FIG. 10B, cores 32 are formed in a predetermined pattern on a surface of the under-cladding layer 31 by a photolithography process. Then, as shown in FIG. 10C, an over-cladding layer photosensitive resin is applied over the cores 32 on the surface of the under-cladding layer 31 to form a photosensitive resin layer 33A. Subsequently, a heat treatment is performed to increase the temperature of the photosensitive resin layer 33A to evaporate a solvent contained in the photosensitive resin layer 33A. At this time, as shown in FIG. 10D, a photosensitive resin component of the photosensitive resin layer 33A penetrates into a surface portion of each of the cores 32 to form a mixed layer 34 composed of a mixture of the photosensitive resin component and the core material. Thereafter, the photosensitive resin layer 33A is exposed by irradiation with radiation, whereby an over-cladding layer 33 is formed from the photosensitive resin layer 33A. Thus, the optical waveguide 30 is produced on the surface of the electric circuit board 20 to provide the opto-electric hybrid board (see, for example, JP-A-2009-103827).
The refractive index of the mixed layer 34 formed in the over-cladding layer forming step is intermediate between the refractive index of the core 32 and the refractive index of the over-cladding layer 33, and is lower than the refractive index of the core 32. Therefore, light in the core 32 is less liable to pass through the mixed layer 34 formed in the surface portion of the core 32, so that the intensity of light transmitted through the mixed layer 34 is much lower than the intensity of light transmitted through a portion of the core 32 inside the mixed layer 34. This prevents scattering of light on a surface of the core 32, thereby reducing the light transmission loss.
However, if components (e.g., information processing components or the like) are connected to each other via a film-shaped connector optical waveguide produced by forming an over-cladding layer in the same manner as the over-cladding layer 33 of the opto-electric hybrid board described above, it is sometimes impossible to properly transmit light between the components.